1. Field of the Invention
This invention generally relates to the fabrication of microelectromechanical systems (MEMS) and, more particularly, to a thin-film diode cantilever MEMS and related fabrication procedures.
2. Description of the Related Art
Active devices such as thin-film transistors (TFTs) and diodes are formed through deposition processes that create thin films of silicon (Si) and insulator material. While the resulting devices may not have the switching speed and drive capability of devices formed on single-crystal substrates, they can be fabricated cheaply with a relatively few number of process steps. Further, thin-film deposition processes permit active devices to be formed on alternate substrate materials, such as transparent glass substrates, for use in liquid crystal displays (LCDs). More specifically, the active devices may include a deposited amorphous Si (a-Si) layer. To improve the performance of the device, the a-Si may be crystallized to form polysilicon, at the cost of some extra processing. The crystallization procedures are also limited by the temperature sensitivity of the substrate material. For example, glass substrates are known to degrade at temperatures over 650 degrees C. Large scaled devices, integrated circuits, and panel displays are conventionally made using thin-film deposition processes.
MEMS devices are a logical derivative of semiconductor IC processes that may be used to develop micrometer scale structural devices such as transducers or actuators. MEMS devices interface physical variables and electronic signal circuits. MEMS structures are varied and, therefore, more difficult to standardize, as compared to the above-mentioned thin film processes. On the other hand, it may be possible to develop MEMS devices by engineering modifications to well-developed silicon IC processes. Many of the MEMS devices that have been fabricated to date have more theoretical than practical application, as the devices are often difficult and expensive to make. For the same reason, larger scale systems using MEMS components have been expensive to fabricate due to the process difficulties and the cost associated with integrating the MEMS and IC technologies.
For example, transistors and associated MEMS structures have been fabricated on bulk Si substrates, and the authors claim excellent performing biochemical sensing MEMS transducers [Vinayak P Dravid and Gajendra S Shekhawat; “MOSFET Integrated Microcantilevers for Novel Electronic Detection of “On-Chip” Molecular Interactions”, Material Science, Northwestern University, Evanston, Ill.]. However, the etching processes needed to form a bulk silicon MEMS are more difficult to control, dramatically limit available process steps, and require long etch times. These limitations make these devices unsuitable for low-cost integrated systems.
Alternately, MEMS structures made using high temperature LPCVD thin films have been built with conventional sensing schemes such as capacitive and/or piezoresistive bridges, generating reasonable output signals [(1) William P. Eaton, James H. Smith, David J. Monk, Gary O'Brien, and Todd F. Miller, “Comparison of Bulk- and Surface-Micromachined Pressure Sensors”, Micromahined Devices and Components, Proc. SPIE, Vol. 3514, P. 431. (2) Joao Gaspar, Haohua Li, Paulo Peieiro Freitas, “Integrated Magnetic Sensing of Electro-statically Actuated Thin-Film Microbridges”, Journal of MicroElectroMechanical Systems, Vol. 12, No. 5, October. 2003, p. 550-556]. However, these sensing schemes cannot be applied to low temperature TFT process, because the changes in electrical characteristics induced as a result of stress change are too small to be practically measured.
Stress is induced on a surface when bio-molecules become immobilized on a solid surface. This property is one of the most promising avenues to explore for bio-sensing. To detect the surface stress, a thin cantilever may be used. The selective absorption or immobilization of molecules on one side of the cantilever creates a surface stress difference between the two sides of the cantilever, and this difference is measured via a change in electrical resistance using an integrated piezoresistor transducer. Alternately, the substantial displacement at the cantilever tip can be detected by an atomic force microscope (AFM). AFM has the best sensitivity, but its expense and complexity prevent it from being widely used. Some key design issues include strain sensitivity and the electrical noise inherent in the sensor. These problems are conventionally addressed by using a single crystal silicon substrate.
Electrically passive piezoresistive cantilever transducers have been studied and demonstrate bio-sensing capabilities for low surface stress sensing. Limited by single crystal silicon anisotropic fabrication processes and relatively poor sensitivity, it is difficult to fabricate a piezoresistive cantilever sensor array at low cost. Prior art devices are usually formed on silicon-on-insulator (SOI) wafers, using high temperature processes and special tools such as deep reactive ion etching (RIE). Bulk micromachining uses a subtractive process to carve the MEMS structure out of the bulk substrate (typically a silicon wafer).
It would be advantageous if a high sensitivity MEMS cantilever could be formed with an integrated active device from laser annealed thin-films, without the necessity of a single crystal silicon substrate or bulk micromachining processes.